PROJECT SUMMARY/ABSTRACT Lung recipients suffer from shorter life spans and higher rejection rates when compared to other solid organ recipients. Bronchiolitis obliterans syndrome (BOS) is the most common cause of chronic lung transplant rejection. Based on clinical outcome studies that have identified many known non-alloimmune mediators of epithelial injury as risk factors for BOS, we have developed a new mouse orthotopic lung transplant model that recapitulates this relationship. This innovation has allowed us to a identify a key airway epithelial cell progenitor, the club cell, as vital for the maintenance of allograft tolerance and the prevention of obliterative bronchiolitis (OB) - the major pathological indicator of BOS. We have also observed that OB is driven by graft-infiltrating effector CD8+ T cells that inhibit club cell-mediated repair of airway epithelium. New data from our laboratory suggests that follicular cytotoxic CD8+ T cells, a poorly defined CD8+ T cell subset with no reported role in solid organ transplantation, may facilitate responses against pulmonary-self antigens. Our group has previously reported that CCR2+ monocytes (CM) and neutrophil extracellular traps (NETs) play critical roles in both lung transplant ischemia-reperfusion injury and the induction of lung allograft tolerance, but their role in the maintenance of tolerance remains unclear. We have now uncovered evidence implicating graft-infiltrating CM and NETs in OB pathogenesis. Here we propose to dissect the mechanisms by which CM (Aim 1), effector CD8+ T cells (Aim 2) and NETs (Aim 3) promote OB pathogenesis. We will also examine how CM and NETs shape effector CD8+ T cell responses that promote OB pathogenesis. Through these studies, we expect to reveal targetable pathways that promote BOS.